1. Field of the Invention
This invention relates generally to a method of sputter deposition of an aluminum-containing film onto a semiconductor substrate, such as a silicon wafer. More particularly, the invention relates to using hydrogen and oxygen ,as with argon during the deposition of aluminum or aluminum alloys to form an aluminum-containing film which is resistant to hillock formation.
2. State of the Art
Thin film structures are becoming prominent in the circuitry components used in integrated circuits (xe2x80x9cICsxe2x80x9d) and in active matrix liquid crystal displays (xe2x80x9cAMLCDsxe2x80x9d). In many applications utilizing thin film structures, low resistivity of metal lines (gate lines and data lines) within those structures is important for high performance. For example with AMLCDs, low resistivity metal lines minimize RC delay which results in faster screen refresh rates. Refractory metals, such as chromium (Cr), molybdenum (Mo), tantalum (Ta), and tungsten (W), have resistances which are too high for use in high performance AMLCDs or ICs. Additionally, the cost of refractory metals is greater than non-refractory metals. From the standpoint of low resistance and cost, aluminum (Al) is a desirable metal. Furthermore, aluminum is advantageous because it forms an oxidized film on its outer surfaces which protects the aluminum from environmental attack, and aluminum has good adhesion to silicon and silicon compounds.
An aluminum film is usually applied to a semiconductor substrate using sputter deposition. Sputter deposition is generally performed inside the vacuum chamber where a solid slab (called the xe2x80x9ctargetxe2x80x9d) of the desired film material, such as aluminum, is mounted and a substrate is located. Argon gas is introduced into the vacuum chamber and an electrical field is applied between the target and the substrate which strikes a plasma. In the plasma, gases are ionized and accelerated, according to their charge and the applied electrical field, toward the target. As the argon atoms accelerate toward the target, they gain sufficient momentum to knock off or xe2x80x9csputterxe2x80x9d atoms and/or molecules from the target""s surface upon impact with the target. After sputtering the atoms and/or molecules from the target, the argon ions, the sputtered atoms/molecules, argon atoms and electrons generated by the sputtering process, form a plasma region in front of the target before coming to rest on the semiconductor substrate, which is usually positioned below or parallel to the target within the vacuum chamber. However, the sputtered atoms and/or molecules may scatter within the vacuum chamber without contributing to the establishment of the plasma region and thus not deposit on the semiconductor substrate. This problem is at least partly resolved with a xe2x80x9cmagnetron sputtering systemxe2x80x9d which utilizes magnets behind and around the target. These magnets help confine the sputtered material in the plasma region. The magnetron sputtering system also has the advantage of needing lower pressures in the vacuum chamber than other sputtering systems. Lower pressure within the vacuum chamber contributes to a cleaner deposited film. The magnetron sputtering system also results in a lower target temperature, which is conducive to sputtering of low melt temperature materials, such as aluminum and aluminum alloys.
Although aluminum films have great advantages for use in thin film structures, aluminum has an unfortunate tendency to form defects, called xe2x80x9chillocksxe2x80x9d. Hillocks are projections that erupt in response to a state of compressive stress in a metal film and consequently protrude from the metal film surface.
There are two reasons why hillocks are an especially severe problem in aluminum thin films. First, the coefficient of thermal expansion of aluminum (approximately 23.5xc3x9710xe2x88x926/xc2x0C.) is almost ten times as large as that of a typical silicon semiconductor substrate (approximately 2.5xc3x9710xe2x88x926/xc2x0 C). When the semiconductor substrate is heated during different stages of processing of a semiconductor device, the thin aluminum film, which is strongly adhered to the semiconductor substrate, attempts to expand more than is allowed by the expansion of the semiconductor substrate. The inability of the aluminum film to expand results in the formation of the hillocks to relieve the expansion stresses. The second factor involves the low melting point of aluminum (approximately 660xc2x0 C.), and the consequent high rate of vacancy diffusion in aluminum films. Hillock growth takes place as a result of a vacancy-diffusion mechanism. Vacancy diffusion occurs as a result of the vacancy-concentration gradient arising from the expansion stresses. Additionally, the rate of diffusion of the aluminum increases very rapidly with increasing temperature. Thus, hillock growth can thus be described as a mechanism that relieves the compressive stress in the aluminum film through the process of vacancy diffusion away from the hillock site, both through the aluminum grains and along grain boundaries. This mechanism often drives up resistance and may cause open circuits.
The most significant hillock-related problem in thin film structure manufacturing occurs in multilevel thin film structures. In such structures, hillocks cause interlevel shorting when they penetrate or punch through a dielectric layer separating overlying metal lines. This interlevel shorting can result in a failure of the IC or the AMLCD. Such a shorted structure is illustrated in FIG. 11.
FIG. 11 illustrates a hillock 202 in a thin film structure 200. The thin film structure 200 comprises a semiconductor substrate 204, such as a silicon wafer, with a patterned aluminum layer 206 thereon. A lower dielectric layer 208, such as a layer of silicon dioxide or silicon nitride, is deposited over the semiconductor substrate 204 and the patterned aluminum layer 206. The lower dielectric layer 208 acts as an insulative layer between the patterned aluminum layer 206 and an active layer 210 deposited over the lower dielectric layer 208. A metal line 212 is patterned on the active layer 210 and an upper dielectric layer 214 is deposited over the metal line 212 and the active layer 210. The hillock 202 is shown penetrating through the lower dielectric layer 208 and the active layer 210 to short with the metal line 212.
Numerous techniques have been tried to alleviate the problem of hillock formation, including: adding elements, such as tantalum, cobalt, nickel, or the like, that have a limited solubility in aluminum (however, this generally only reduces but not eliminates hillock formation); depositing a layer of tungsten or titanium on top or below the aluminum film (however, this requires additional processing steps); layering the aluminum films with one or more titanium layers (however, this increases the resistivity of the film); and using hillock resistant refractory metal films such as tungsten or molybdenum, rather than aluminum (however, as previously mentioned, these refractory metals are not cost effective and have excessive resistivities for use in high performance ICs and AMLCDs).
In particular with AMLCDs and, more particularly, with thin film transistor-liquid crystal displays (xe2x80x9cTFT-LCDsxe2x80x9d), consumer demand is requiring larger screens, higher resolution, and higher contrast. As TFT-LCDs are developed in response to these consumer demands, the need for metal lines which have low resistivity and high resistance to hillock formation becomes critical.
Therefore, it would be advantageous to develop an aluminum-containing material which is resistant to the formation of hillocks and a technique for forming an aluminum-containing film on a semiconductor substrate which is substantially free from hillocks, while using inexpensive, commercially-available, widely-practiced semiconductor device fabrication techniques and apparatus without requiring complex processing steps.
The present invention relates to a method of introducing hydrogen and oxygen gas, along with argon gas, into a sputter deposition vacuum chamber during the sputter deposition of aluminum or aluminum alloys onto a semiconductor substrate, including but not limited to glass, quartz, aluminum oxide, silicon, oxides, plastics, or the like, and to the aluminum-containing films resulting therefrom.
The method of the present invention involves using a standard sputter deposition chamber, preferably a magnetron sputter deposition chamber, at a power level of between about 1 and 4 kilowatts (KW) of direct current power applied between a cathode (in this case the aluminum target) and an anode (flat panel display substratexe2x80x94i.e., soda lime glass) to create the plasma (after vacuum evacuation of the chamber). The chamber is maintained at a pressure of between about 0.5 and 2.5 millitorr with an appropriate amount of argon gas, hydrogen gas, and oxygen gas flowing into the chamber. The argon gas is preferably fed at a rate between about 25 and 90 standard cubic centimeters per minute (xe2x80x9csccmxe2x80x9d). The hydrogen gas is preferably fed at a rate between about 50 and 400 sccm. The oxygen gas is preferably fed at a rate between about 0.25 and 2 sccm (preferably in an atmospheric air stream). The ratio of argon gas to hydrogen gas is preferably between about 1:1 and about 1:6. The films with higher hydrogen/argon ratios exhibited smoother texture than lower hydrogen/argon ratios. The deposition process is conducted at room temperature (i.e., about 22xc2x0 C.).
The aluminum-containing films resulting from this method have an average oxygen content between about 12 and 30% (atomic) oxygen in the form of aluminum oxide (Al2O3) with the remainder being aluminum. The aluminum-containing films exhibit golden-yellow color when formed under the process parameters described. The most compelling attribute of the aluminum-containing films resulting from this method is that they are hillock-free, even after being subjected to thermal stresses.
Although the precise mechanical and/or chemical mechanism for forming these aluminum-containing films is not completely understood, it appears that the hydrogen gas functions in the manner of a catalyst for delivering oxygen into the aluminum-containing films. Although the flow of the oxygen gas into the vacuum chamber is small compared to the flow of argon gas and hydrogen gas, there is a relatively large percentage of oxygen present in the deposited aluminum-containing films. In experiments by the inventors, oxygen gas was introduced into the vacuum chamber without any hydrogen gas being introduced (i.e., only oxygen gas and argon gas introduced). The resulting films deposited on the substrate did not have a measurable amount (by x-ray photoelectron spectroscopy) of oxygen present.
As stated previously, oxygen is present in the deposited aluminum-containing film in the form of aluminum oxide. However, aluminum oxide is an insulator. It is counter-intuitive to form an insulative compound (which should increase the resistivity of the film) in a film which requires very low resistivity. However, it has been found that the formation of the aluminum oxide does not interrupt the conducting matrix of aluminum grains within the aluminum-containing film. Thus, the resistivity of the aluminum-containing film is surprisingly low, in the order of between about 6 and 10 micro ohm-cm. This is particularly striking in light of the fact that aluminum oxide is present in the range of between about 12 and 30% (atomic). The grain size of these aluminum-containing films is between about 400 and 600 angstroms (xc3x85).
Aside from being substantially hillock-free and having a low resistivity (i.e., high conductivity), the resultant aluminum-containing films have additional desirable properties including low roughness, low residual stress, and good mechanical strength (as determined by a simple scratch test compared to pure aluminum or by the low compressive stress (between about xe2x88x925xc3x97108 and xe2x88x921xc3x97109 dyne/cm2), which is considered to be an indication of high scratch resistance). Measurements of the aluminum-containing films have shown that the roughness before and after annealing is low compared to pure aluminum (about 600-1000 xc3x85 before annealing and 400-550 xc3x85 after annealing). Low roughness prevents stress migration, prevents stress-induced voids, and, consequently, prevents hillock formation. Additionally, low roughness allows for better contact to other thin films and widens the latitude of subsequent processing steps, since less rough films result in less translation of crests and valleys in the film layers deposited thereover, less diffuse reflectivity which makes photolithography easier, no need to clad the aluminum in the production of AMLCDs (rough aluminum traps charge which effects electronic performance [i.e., high or variable capacitance]), and more uniform etching.
The mechanical strength of the aluminum-containing films resulting from the process of the invention is higher than conventionally sputtered thin films of aluminum and some of its alloys. A high mechanical strength results in the resulting aluminum-containing films being resistant to both electromigration and stress induced voiding.
This combination of such properties is superior to that of thin films of aluminum and its alloys which are presently known. These properties make the aluminum-containing films of the present invention desirable for electronic device interconnects. These properties are also desirable in thin films for optics, electro-optics, protective coatings, and ornamental applications.